The invention is for example intended for producing hollow bodies, such as containers, starting from preforms made of plastics material in a so called “hydraulic forming process” or a “blow molding forming process”, like blow molding or injection stretch blow molding (ISBM) with a machine adapted to load the preforms in successive molds, to inject a gaseous or liquid fluid in performs heated above the glass temperature of the plastics material, in order to deform the preforms such that they acquire the shape of the molding cavity, and to unload the formed containers from the molds. Conventionally, such machines perform various operations, such as the forming operation or a capping of the containers operation in different stations each having a wheel, or carousel, movable in rotation carrying the molds or the preforms or the containers in order to have a continuous process having a satisfactory throughput.
In such machines and processes, the molds have to be opened frequently to load the preforms and unload the formed containers, while at the same time needing to be firmly locked in the closed configuration in order to prevent the molds from opening during the injection of the fluid at high pressure or during the movement of the molds when they are carried by a wheel at a station of the machine.
In conventional machines, it has been proposed that mold holders attached to the wheel preform the operations of opening, closing and locking the molds. Each mold holder is consequently arranged to open the mold it carries in order to allow the loading of the preform inside the mold, and comprises or is associated with means to close and lock the mold once the preform is loaded, which means are able to maintain the mold closed and locked during the rotation of the wheel and the forming of the container, and finally to unlock and open the mold once again to allow the retrieving of the formed container from the mold. Such mold holders are therefore structurally complicated to enable the opening and the closing of the molds and comprise many parts and recesses. Furthermore, in case of hydraulic forming, the design of the mold holders is constrained due to the fact that they have to be made in stainless steel or any other non-corrosive material or coating, to cope with the wet environment and the necessary cleaning procedures. The mold holding units also have to withstand the inertia at high revolution speed, which not only depends of the wheel diameter, but also on the increased weight compared to air forming because of the product contained in the preforms and formed containers. Furthermore, hydraulic forming allows high forming pressures, for example as high as 50 bars, thus creating a very good print of the container formed, but also creating a very high level of mechanical constraints on the mold holding units. The mechanical shocks due to opening and closing of the mold holders also have to be considered when designing these units. All these restrictions make the mold holders very cumbersome, which limits the number of mold holders carried by the wheel and increases the distance, or pitch, between successive mold holders on the wheel.
Therefore, in order to have a satisfactory throughput at the forming station, it is necessary to increase the speed of rotation of the wheel, which increases the centrifugal force in the wheel and can lead to spilling the fluid injected in the preforms. Indeed, if peripheral speed is too high, at the end of the forming process, when the forming nozzle is moved up, centrifugal force may force the liquid to spill on top of the mold cavities. This is particularly problematic in case of hydraulic forming because the spilled liquid will soil the mold and the mold holder. The mold holder is hard to clean due to its complicated structure, and cleaning the mold holder implies stopping the rotation of the wheel and therefore reduces the throughput of the forming station. An insufficient cleaning of the molds and mold holders can lead to cross-contamination in the hydraulic forming method. Consequently, the speed of rotation of the wheel is necessarily limited in order to avoid the above-mentioned problems.
The same problems may rise in other fields than the forming of containers, as soon as a hollow body is to be produced from a preform placed in a mold.
One of the aims of the invention is to solve the above-mentioned problems by proposing a mold and a machine of simpler structures, the mold being adapted to cooperate with a more compact and simpler mold holder.